Yale C/AIM Web Style Guide


Web Graphics


Color display primer

Graphic file formats

Interlaced GIF

Transparent GIF

JPEG graphics

Summary-File formats


Optimizing graphics I

Optimizing graphics II

Height & width tags

Colored backgrounds


GIF files
The Graphic Interchange Format (GIF) was popularized by the Compuserve Information Service in the 1980s as an efficient means to transmit images across data networks. In the early 1990s the original designers of the World Wide Web adopted the GIF format for its efficiency and widespread familiarity. Today the overwhelming majority of images on the Web are in GIF format. Virtually all Web browsers that support graphics support the GIF file format for inlined images. The GIF format incorporates a compression scheme to keep files sizes at a minimum, and GIF files are limited to 8-bit (256 or fewer colors) color palettes. There are now several slight variants of the basic GIF file format that add support for transparent color, and support for the interlaced GIF graphics popularized by the Netscape Navigator Web browser.
You may see references to the different GIF formats, such as "GIF87a," or "GIF89a." All forms of GIF images will work in Web browsers that support the basic GIF file format, so that you do not have to worry whether your readers will be able to see your GIF graphics, regardless of the GIF version that you use. Users whose browsers support the transparency and interlacing (such as Netscape Navigator and Microsoft Explorer) will see more sophisticated visual effects, but everyone will see your basic GIF images.

GIF File Compression
The GIF file format uses a relatively basic form of file compression (Lempel Zev Welch, or LZW) that squeezes out inefficiencies in the data storage without causing a loss of any data ("lossless compression") or distortion of the image. The LZW compression scheme is most efficient at compressing images with large fields of homogeneous color. It is not very good at squeezing complex pictures with lots of grainy texture. All variations of the GIF graphics file format incorporate LZW file compression. See Siegel (1996) for an excellent discussion on optimizing graphics for GIF compression.

GIF compression examples.


Siegel, D. 1996. Creating killer web sites. Indianapolis: Hayden Books.


Weinman, L. 1996. Designing Web graphics. Indianapolis: New Riders.

Copyright 1997 P. Lynch and S. Horton,
   all rights reserved. Yale University   Revised January 1997.